This invention relates to a brazing method for joining component parts of aluminium or alloys thereof.
A flux employed today for joining the aluminium parts with a brazing metal could be classified into two groups, viz., one which is soluble in water and another which is insoluble in water. The water-soluble flux is generally made from a mixture of chlorides such as chloride of alkali metal or alkaline earth metal, while the water-insoluble flux is represented by fluorides such as fluoroamine-potassium-complex. Almost all of the water-soluble fluxes which are made from chlorides are deliquescent, and when they become moist, they have severe corrosive actions against aluminium materials and brazing apparatus. On the other hand, residues of fluxes containing fluorides which have adhered to surfaces of aluminium parts after brazing, can hardly be removed by washing.
For example, a conventional water-solute flux which is made from a mixture of chlorides such as NaCl, KCl, LiCl, ZnCl.sub.2, CaCl.sub.2, and fluorides is extremely deliquescent. When said flux is applied to a brazing sheet, and heated in a furnace, it is decomposed and evaporates as shown in the following formula, whereby its efficiency as flux considerably lowers. EQU ZnCl.sub.2 +H.sub.2 O.fwdarw.ZnO+HCl
Meanwhile, the flux which has evaporated, adheres to the furnace and its fixtures and jigs, adsorbs moisture, and corrodes them. This means that a brazing operation with the employment of a deliquescent flux makes it meaningless to carry it under an atmosphere of a low oxygen and low dew point such as of an oxygen content lower than 50 ppm and a dew point lower than -50.degree. C., since the flux itself is hydroscopic. Therefore, the operation is generally conducted in an atmospheric condition. Since this condition is not reliable to prevent the oxidation of component parts of aluminium or alloys thereof, an excessive amount of flux has to be used for the brazing operation in order to remove metal oxides produced during the operation. Further, residues of flux which have adhered to the components have to be throughly washed out, since they are corrosive against the components in the presence of moisture.
The aforementioned water-insoluble fluxes have been developed so that it is not necessary they be removed after brazing. They are mixtures of KAlF.sub.4 and K.sub.3 AlF.sub.6 which are insoluble in water even after brazing, and are not corrosive even in the presence of moisture. However, this kind of flux has the drawback that its residues adhere firmly onto aluminium components after brazing, and they can hardly be removed. Especially when the components are to be subjected to a surface treatment after having been brazed, the residues become barriers to the treatment. And, when brazed aluminium components are employed in processing food, it is impossible to prevent scattering of flux residues into the food, though this should be completely avoided.